fix(ssh): preserve managed server ownership and install failures

[rynfar]

SSH-managed servers launched through npm wrappers can have the wrong recorded PID, breaking graceful shutdown and leaving ports occupied. Exec the resolved CLI directly after installation, and preserve npm installation failures instead of misreporting them as missing build tools or running a path printed by a failed command.

Adopts upstream #9843 and #10088 as one runner-fix series. Pylon package selection, runtime home, and identity stay intact; the partial adoption is recorded in the ledger.

Validation: 33 runner/tunnel tests, SSH package typecheck, targeted lint and formatting pass. Real fixture processes verify PID ownership, graceful shutdown, port reuse, argument quoting, and npm/npx diagnostics. Unix process fixtures are skipped on Windows. No live environment was touched.

The durable implementation and review record is consolidated in companion #279.

Model: GPT-6 Astra. Harness: Codex.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.8 KiB	+149 B (+1.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−4 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.7 KiB	+153 B (+2.3%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	57.3 KiB	+866 B (+1.5%)	66.4 KiB	✅
Codex	Live turn messages	8	9	+1 (+12.5%)	21	✅
Claude	Total thread wire	13.6 KiB	13.8 KiB	+120 B (+0.9%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−8 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.7 KiB	+128 B (+1.9%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.3 KiB	58.2 KiB	+884 B (+1.5%)	66.4 KiB	✅
Claude	Live turn messages	8	9	+1 (+12.5%)	21	✅

Baseline: de8d090 · PR result: b20c9d6 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.